1. Field of the Invention
The present invention relates to rotary connectors including a stator housing and a rotor housing that are rotatably coupled and electrically connected through a flexible cable, and particularly relates to a rotary connector including a rotor housing composed of upper and lower rotors coupled and integrated with, for example, a snap fit.
2. Description of the Related Art
Rotary connectors include, for example, a stator housing fixed to a combination switch assembly provided on a steering unit of an automobile, a rotor housing attached to a steering wheel, and a flexible cable wound in an annular accommodation space defined between the stator housing and the rotor housing. These rotary connectors are used to provide electrical connection for, for example, an airbag inflator provided on a steering wheel, which has a limited number of revolutions.
Among such rotary connectors is a known rotary connector according to, for example, Japanese Unexamined Patent Application Publication No. 2002-58150 (see Pages 5 and 6 and FIG. 2 of the publication). This rotary connector includes a rotor housing composed of upper and lower rotors which are integrated with a snap fit in the final stage of its assembly process so that the rotor housing is rotatably attached to a stator housing. FIG. 3 is a sectional view of an example of such a known rotary connector. This rotary connector mainly includes a stator housing 20, a rotor housing 21 rotatably attached to the stator housing 20, a holder 23 rotatably disposed in an annular accommodation space 22 defined between the housings 20 and 21, and a strip-like flat cable 24 wound and accommodated in the accommodation space 22.
The stator housing 20 includes an outer cylindrical portion 25 and a bottom cover 26 that are made of a synthetic resin and are integrated with, for example, a snap fit. A control wall 25a protrudes inward from the top end of the outer cylindrical portion 25, and a circular central hole 26a is formed in the center of the bottom cover 26. The rotor housing 21 includes an upper rotor 27 and a lower rotor 28 that are made of a synthetic resin. The upper rotor 27 includes an annular top plate 27a and an inner cylindrical portion 27b extending downward from the inner edge of the top plate 27a. The top plate 27a and the inner cylindrical portion 27b are integrally formed. The lower rotor 28 includes a cylindrical wall 28a and a flange 28b protruding outward from the bottom end of the cylindrical wall 28a. The cylindrical wall 28a and the flange 28b are integrally formed. The cylindrical wall 28a is snapped into the inner cylindrical portion 27b to integrate the upper rotor 27 and the lower rotor 28. In the integration of the upper rotor 27 and the lower rotor 28, the bottom surface of the outer edge of the top plate 27a of the upper rotor 27 is brought into contact with the top end of the control wall 25a of the outer cylindrical portion 25 while the flange 28b of the lower rotor 28 is brought into contact with the bottom surface of the edge of the bottom cover 26 around the central hole 26a. As a result, the rotor housing 21 is rotatably attached to the stator housing 20.
The accommodation space 22 is defined by the outer cylindrical portion 25 and bottom cover 26 of the stator housing 20 and the top plate 27a and inner cylindrical portion 27b of the rotor housing 21. This accommodation space 22 accommodates the holder 23 and the flat cable 24. The holder 23 includes rollers 23a and an annular rotary plate 23b. The rotary plate 23b is made of a synthetic resin and is rotatably placed on the top surface of the bottom cover 26. The rollers 23a are rotatably supported on the top surface of the rotary plate 23b, and openings of a predetermined size are defined between the rollers 23a adjacent in the circumferential direction. The accommodation space 22 accommodates the flat cable 24 with its winding direction reversed at any point of its length. The flat cable 24 turns around any of the rollers 23a on the holder 23. Lead blocks (not shown) are connected to the longitudinal ends of the flat cable 24. These lead blocks are fixed to predetermined positions of the outer cylindrical portion 25 and the upper rotor 27 so that the flat cable 24 is electrically connected to the outside of the housings 20 and 21.
For the rotary connector having the above structure, if the rotor housing 21 (the upper rotor 27 and the lower rotor 28) is rotated in either a forward or reverse direction with respect to the stator housing 20 (the outer cylindrical portion 25 and the bottom cover 26), the reversed portion of the flat cable 24 moves in the same direction by a smaller amount of rotation than the upper rotor 27, and the holder 23 moves in the same direction accordingly. As a result, the flat cable 24 is unwound from the inner cylindrical portion 27b of the upper rotor 27 to the outer cylindrical portion 25 or is wound around the inner cylindrical portion 27b of the upper rotor 27 from the outer cylindrical portion 25 by a length about twice the amount of movement.
The above known rotary connector holds the stator housing 20 and the rotor housing 21 in the axial direction by bringing the bottom surface of the outer edge of the top plate 27a into contact with the top end of the control wall 25a and the flange 28b of the lower rotor 28 into contact with the bottom surface of the edge of the bottom cover 26 around the central hole 26a. The rotor housing 21 may therefore be smoothly rotated with the contact surfaces sliding over each other if the dimensional accuracy of each member is maintained. However, the dimensional accuracy of the molded components constituting the stator housing 20 and the rotor housing 21 tends to decrease because the sliding surfaces of the top plate 27a and the control wall 25a are separated from the center of rotation of the rotor housing 21 by a large distance. In addition, the top plate 27a readily suffers, for example, dimensional variations and warping due to changes in ambient temperature. As a result, unfortunately, the sliding portions of the top plate 27a and the control wall 25a generate unusual noise when the rotor housing 21 is rotated.